Acute lung injury (ALI) and its most severe presentation acute respiratory distress syndrome (ARDS), represent a full spectrum of a complex and devastating illness, with associated mortality hovering at 30[unreadable] 40%. Even supplemental O2, a routine and needed therapy for such patients, paradoxically causes lung injury. In fact, the detrimental effects of O2 have established hyperoxic acute lung injury (HALI) as a prototype to study respiratory distress syndromes in experimental animals. To confront the high ALI mortality much differently than current candidate gene studies, we have established a mouse model (sensitive C57BL/6J and resistant 129X1/SvJ mice) to assess the genetic complexity of HALI, with a longterm goal to identify genes affecting strain survival differences. Segregation analysis of 840 F2 mice generated from the four possible intercrosses between these strains verified that survival time is a complex trait with decreased penetrance, and significant sex, cross, and parent-of-origin effects. Quantitative trait locus (QTL) analyses of the 840 F2 mice identified three highly significant loci (named Shali1[unreadable]3, for Survival to hyperoxic acute lung injury) and one significant locus (Shali4) in the total F2 population. Pairwise analysis identified several additive gene-gene interactions amongst the QTLs and an epistatic interaction with an otherwise unlinked locus. Segregation and QTL analyses demonstrated that resistance alleles originate from both parental strains and recombine to determine individual HALI susceptibility. These results have led to the following hypothesis: Shali QTLs contain susceptibility genes that, when grouped together in appropriate allelic combinations, will significantly affect HALI survival time. The primary objective of this application is to set the stage for physical mapping and quantitative trait gene identification, with a major focus on Shali1. To accomplish this, we propose three Specific Aims: 1) confirm QTL results in vivo by constructing reciprocal congenic strains for the 4 Shali QTLs significantly linked to HALI survival time in the B[unreadable]S model; establish which QTL(s) significantly contribute to HALI survival time; 2) test candidate genes and reduce the Shali1 QTL interval to a level amenable to physical mapping; prioritize and critically assess positional candidate genes for functional significance; concurrently reduce the Shali1 QTL interval by constructing and testing congenic sub-strains; and 3) determine the best allelic combinations for increased and decreased survival in multi-congenic strains containing the corresponding QTLs in the appropriate strain; generate reciprocal congenics with all 4 sensitive or all 4 resistant Shali alleles in the same strain. Mouse lines derived from these studies will provide impetus and focus towards identifying key genes affecting HALI survival.